1. Field of the Invention
The present invention generally relates to an optical coupling apparatus (or optical coupler) and a method of manufacturing the same, a light emitting diode package and a method of assembling the same and a lens holder. More particularly, the invention is concerned with a light emitting diode/receiver apparatus as well as constituent parts thereof employed in optical communication systems, optical recording apparatus, optical measurements and others.
2. Description of the Prior Art
In the light emitting diode/receiver apparatus incorporating a microlense, the latter microlense is fixedly secured on a block by using a resin or is hermetically mounted on a cap, as is disclosed in JP-A-61-20911 (Japanese Patent Application Laid-Open No. 20911/1986), JP-A-61-134709 and JP-A-63-19609. However, a disadvantage the known light emitting diode/receiver apparatus resides in the face that, upon mounting the lens, the optical axes of the light emitting element and the lenses are changed or misaligned due to inaccurate positioning of the lens mounting block. Moreover, there is a nonuniformity in the layer thickness of the resin, contraction of the resin due to curing.
Further, the cap is secured along the periphery thereof by a laser welding or soldering with a view to avoiding the use of resin which is likely to provide a source for contamination. However, this type bonding frequently resulting in positional misalignment because of the necessary hardening process.
Thus, it has been extremely difficult to optically align the positions of the light emitting element, for example a laser element, and an associated lens in accordance with predetermined design specifications.
In the design of a light emitting diode package for optical communication, accuracy required for positional alignment is in a range of a few .mu.m in the direction perpendicular to the optical axis. Upon occurrence of misalignment in the lens position, the optical axis of the lens is inclined relative to a predetermined or aimed optical axis, resulting in light rays, emitted by a light emitting element such as a semiconductor laser diode, being incident on the lens with inclination, whereby the quantity or amount of light introduced into the optical fiber is significantly reduced. In the above-described construction, when the lens is fixedly mounted on a rigid mount or within a lens case, the lens is rigidly secured, making it impractical to correct or compensate for the misalignment of the optical axis which may take place upon bonding.
When a lens is to be mounted in opposition to a laser element, the laser element is first fixed in a package. Subsequently, the lens is secured at a predetermined distance from the laser element. The accuracy required in aligning the optical axes of both elements (that is the light emitting element and the lens), is in the submicron range. For fixation by bonding, a resin, solder or the like may be employed. When the lens is fixed or secured in position by using this type of bonding material, deformation in a submicron range is likely to take place due to contraction upon curing or setting, thermal deformation or the like, and such deformation can not be neglected. The deformation necessarily resulting in misalignment of the optical axis and degradation in the optical coupling efficiency.
In the apparatuses disclosed in JP-A-61-20911 and others, a lens holder is packaged or secured to a base plate for allowing the lens position to subsequently be finely adjusted. It is however, noted that even when the lens holder is secured after positional alignment to a rigid member forming a part of the main body of the light emitting unit or apparatus, posterior positional misalignment in a submicron range will inevitably occur. Even with the structure disclosed in JP-A-63-19609 in which the lens holder is adapted to be movable along the optical axis, there is no suggestion at all as to the possibility of fine adjustment of the lens position relative to the optical axes common to the lens and the light emitting element.
As the optical coupling means to be employed in the optical coupling apparatus which incorporates an optical isolator, there is often employed a compound lens system including a plurality of lenses. In order to assure a high optical coupling efficiency in this type optical coupling apparatus, a great emphasis is placed on a method of decreasing aberrations of the lens and adjustments of the individual optical elements such as semiconductor laser diode, lenses, optical fiber and others.
A typical example of this type optical coupling package is described in a Japanese periodical "MITSUBISHI DENKI GIHOU", Vol. 62,No. 10 (1988), with. FIG. 22 of the accompanying drawing illustrating a sectional view this known optical coupling package.
Referring to the FIG. 22, a light emission element (semiconductor laser diode or LD) 61 is mounted to a chip carrier 58, and a first lens (spherical lens) 62 is fixedly disposed at the front. A thermistor 59 and a monitoring element 60, such as example a photodiode and provided with the first lens 62 being adjustable so as to be positioned in the y- and z-directions. However, the position of the first lens 62 in the x-direction is determined by the thicknesses of the thermoelectric cooler 57 and the chip carrier 58 and is not subject to positional adjustment. Light rays emitted by the light emitting diode 61, collected by the first lens 62, pass through an optical isolator 63 along the center axis thereof and are collected by a second lens 64 so as to be introduced to a single-mode optical fiber 65.
The optical isolator 63 and the second lens 64 are fixedly assembled within a cylindrical pipe or tube and can not be adjusted in the x- and y-directions. Accordingly, the coupling efficiency of the optical coupling package disclosed in the publication cited above is essentially determined by the machining precision of the individual optical elements. Assuming, by way of example, that the semiconductor laser diode and the optical fiber (single-mode optical fiber) are to be optically coupled by using two rod lenses, the alignment offset of .+-.2.5 .mu.m in the x- and/or y-direction will result in typically 1 dB degradation in the coupling loss. Under these circumstances, construction permitting a final entered adjustment of the optical coupling package is required.
Although the reliable method of bonding the first lens 62 in the optical coupling package is not disclosed in the abovementioned publication, it is perceived that the first lens 62 is mounted to the chip carrier 58 by using a solder or other brazing metal. However, fixation of the first lens by using a brazing metal after adjustment of the optical axis may sometimes be accompanied by a fine positional misalignment or deviation due to contraction upon curing of the brazing metal or due to residual stress in the aging process, which is, of course, detrimental to realization of a sufficiently high optical coupling efficiency in the disclosed optical coupling package.
Additionally, in the prior techniques mentioned above, no consideration is given to the method for fine adjustment of the optical axes of the semiconductor laser, lens, optical isolator and the optical fiber. In this package design, it must again be pointed out that satisfactory or sufficient optical coupling can not be obtained with the assembly in which attention is only paid to the machining precision of the individual optical elements. Further, once the lens, one of the optical elements, is mounted to the chip carrier by using a brazing metal after adjustment of the optical axis, the portional misalignment to which the lens is fixed can not easily be modified for correcting the off-axis deviation of the lens which may be brought about due to contraction of the brazing metal occurring upon curing or setting thereof. In other words, it is impossible to readjust the optical axis of the lens and hence that of the optical coupling package.